DC voltage sources are widely used as power supplies in many electronic devices. Generally, the DC voltage supplied by the DC voltage source is derived from an AC voltage. The AC voltage is rectified into an unregulated DC voltage by a rectifier firstly. Then the unregulated DC voltage is converted into a stable DC voltage as needed by the electronic devices.
Generally, a switching regulator comprises an energy storage component and a main switch which is usually electronically coupled to the energy storage component. The main switch is turned ON and OFF to alternately store energy in the energy storage component and transfer the stored energy to a load. The energy storage component may be realized by a transformer or an inductance.
There are two primary types of control methods used in the switching regulator. One is fixed frequency control and the other is variable frequency control. Although fixed frequency control is more widely applied, it suffers from high switching loss and efficiency variation with load or input voltage.
An example of variable frequency control is quasi-resonant (QR) control. FIG. 1 shows an example waveform of a switching regulator under QR control, where Vs represents the voltage across the main switch, CTRL represents a control signal controlling the ON and OFF of the main switch, and It represents the current flowing through the energy storage component. In the example of FIG. 1, the switching regulator works under DCM (discontinuous current mode). When the current It flowing through the energy storage component reduces to zero, the energy storage component resonates with the parasitic capacitance of the main switch. The main switch is turned ON when the voltage Vs across the main switch reaches its resonant valley, so as to reduce switching loss. The main switch is turned OFF when the current It flowing through the energy storage component reaches a threshold level, which in the example of FIG. 1 may be a peak current limit signal.
Under QR control, the lighter the load, the shorter the ON time period and OFF time period of the main switch. If the output voltage is fixed, the light load and the high input voltage may result in high switching frequency and the consequent EMI (electromagnetic interference) problem. The EMI may not only reduce the quality of the power network, but also influence electrical devices connected to or placed near the switching regulator. Therefore, the switching frequency should be limited, for example, to be lower than 150 kHz.
The switching frequency may be limited by setting a time period limit, such as a minimum switching period, a minimum ON time period or a minimum OFF time period of the main switch. If a minimum OFF time period of the main switch is set, the main switch is only turned ON at the resonant valley after the minimum OFF time period. Thus the switching frequency is limited while the valley switching feature is preserved. However, this frequency limitation method may cause audible noise due to the frequency hopping.
FIG. 2 shows waveforms of signals in a conventional QR controlled switching regulator with frequency limitation, where Tmin represents the minimum OFF time period, and point A represents a resonant valley of the voltage Vs across the main switch. In real world applications, the position of the resonant valley A may vary due to disturbances in the circuit. When the resonant valley point A is slightly earlier, which means it occurs inside the minimum OFF time period Tmin, the main switch will be turned ON at the next resonant valley. When the resonant valley point A is slightly later, which means it occurs outside the minimum OFF time period Tlimit, the main switch will be turned ON at the resonant valley point A. So the OFF time period of the main switch may vary due to the disturbances even when the load and the input voltage are stable. The variation of the OFF time period of the main switch will cause the switching frequency to hop in several switching periods, which may generate low frequency audible noise.
There are several methods of changing the minimum OFF time period Tlimit to avoid the switching frequency hopping. One method is to set two minimum OFF time periods. When one minimum OFF time period is close to the resonant valley, the system adopts the other minimum OFF time period to make sure there is enough distance between the minimum OFF time period and the resonant valley, so as to eliminate the risk of switching frequency hopping. The disadvantage of this method is that it is difficult to find two minimum OFF time periods suitable to a system under any conditions.
The present disclosure provides a quasi-resonant controlled switching regulator adopting a varying minimum OFF time period.